1. Field of the Invention
This invention relates to a magnetic recording disk comprising a cartridge casing and a magnetic recording disk accommodated therein for rotation, the magnetic recording disk consisting of an annular magnetic recording medium and a center core secured to the recording medium at the center thereof.
2. Description of the Prior Art
Magnetic recording disks are widely used as sound and/or video recording media in computers, word-processors and the like. Recently, a miniaturized magnetic recording disk sometimes referred to as a "micro floppy disk" is used in a so-called electronic camera instead of silver salt film. Especially in the miniaturized magnetic recording disk cartridge, the recording density has to be very high since a large amount of information must be recorded on a small recording medium. Therefore, slight deformation of the recording medium or slight displacement of the recording medium relative to the center core with which the driving mechanism of the recording/reproducing system is engaged can adversely affect recording and/or reproduction of information.
As shown in FIG. 1, the magnetic recording disk 1 generally comprises an annular magnetic recording medium 2 formed of an annular flexible base sheet having a central opening 2a and bearing thereon a magnetic recording layer, and a center core attached to the recording medium 2 concentrically with the central opening 2a. The center core comprises an upper half 3 and a lower half 4 secured together by means of, for instance, ultrasonic welding so as to sandwiched therebetween the magnetic recording medium 2 with a resilient ring 5 intervening between the upper half 3 and the upper surface of the recording medium 2. Displacement of the recording medium 2 with respect to the center core in the circumferential direction thereof is limited by surface friction of the resilient ring 5. However this method of securing the center core to the magnetic recording medium has been found to be disadvantageous in that the resilient ring 5 is strained upon ultrasonic welding, whereby deformation such as warpage is generated in the recording medium 2 or the recording medium is displaced in the circumferential direction with respect to the center core. In order to solve this problem, it has been proposed to secure the recording medium 2 to the lower half 4 of the center core by applying double-sided adhesive tape therebetween instead of disposing the resilient ring 5 between the upper half 3 and the recording medium 2. This method is advantageous in that the problem that positioning of parts in assembly is difficult due to the fact that the parts are light in weight and small in size is solved, thereby permitting automated assembly of the magnetic recording disk cartridge. On the other hand, it is disadvantageous in that the standard of 0.5.+-.0.02 mm for the distance between the upper surface of the recording medium 2 and the lower surface of the flange 4a of the lower half 4 cannot be satisfied since the double-sided adhesive tape is of an unwoven fabric impregnated with adhesive and has a thickness of 50 to 150 .mu.m.